Role of oxidative carbonylation in protein quality control and senescence

Abstract

Proteins can become modified by a large number of reactions involving reactive oxygen species. Among these reactions, carbonylation has attracted a great deal of attention due to its irreversible and unrepairable nature. Carbonylated proteins are marked for proteolysis by the proteasome and the Lon protease but can escape degradation and form high-molecular-weight aggregates that accumulate with age. Such carbonylated aggregates can become cytotoxic and have been associated with a large number of age-related disorders, including Parkinson's disease, Alzheimer's disease, and cancer. This review focuses on the generation of and defence against protein carbonyls and speculates on the potential role of carbonylation in protein quality control, cellular deterioration, and senescence.

Figure 1

Carbonylation and derivatization of a protein amino-acid side chain. A scheme for the formation of glutamic semialdehyde from an arginyl residue is depicted as a consequence of an MCO. For detection, the carbonyl group, in this case glutamic semialdehyde, is subsequently derivatized by 2,4-dinitrophenol hydrazine. The resulting protein 2,4-dinitrophenol hydrazone can be detected by specific monoclonal or polyclonal antibodies (see Requena et al, 2001).

Figure 2

Schematic representation of pathways that can generate carbonylation-prone protein species and the fate of such proteins. Pathway 1 portrays the generation of aberrant proteins by mistranslation, for example, missense incorporation, frame-shifting, and stop-codon read-through. Pathway 2 signifies the formation of aberrant folding structures during chaperone deficiency, whereas pathway 3 is the formation of similar aberrant structures during specific stresses such as heat and exposure to denaturing agents. Pathway 4 illustrates the hypothetical carbonylation of an idle enzyme structure, which prevails during substrate deficiency. As long as the substrate is present and the enzyme working, it may be safeguarded from carbonylation and less prone to be attacked by proteases. Soluble species of carbonylated proteins appear ordained for proteolysis by, for example, the proteasome or Lon protease. However, highly carbonylated proteins form high-molecular-weight aggregates that are proteolysis-resistant. Such aggregates appear to inhibit protease functions.

Figure 3

Segregation of carbonylated proteins during cytokinesis of the yeast S. cerevisiae. (A) Transmission electrograph of a yeast mother cell in the process of generating a daughter cell. (B) Levels of superoxide as determined by DHE staining in cells shown in panel A. (C) In situ detection of carbonylated proteins (see Aguilaniu et al, 2003) of cells shown in panel A.